The present invention relates generally to improvements in resilient electroconductive members, and it relates more particularly to an improved electroconductive elastic sponge member having an electrical resistivity in the range of 10.sup.2 to 10.sup.6 ohm/cm and possessing fine pliability and fine compression restorability, and it relates further to an electroconductive elastic sponge member for use in the field of electrophotography, such as in an electrostatic latent image transfer mechanism.
Conventionally, an electroconductive member in the form of a plate, roller or belt has been used in devices for electrophotography, printing, spinning and the likes for antistatic and for electrostatic charge injection purposes. Specifically in electrophotographic copying machines, an electroconductive member is used to remove excess electrostatic charge from a copy paper and/or to transfer an electrostatic latent image formed on a suitable photosensitive plate or substrate to a copy paper. Such electroconductive members are commonly prepared by coating or kneading an antistatic agent mainly comprising an anionic, cationic, or nonionic surfactant on an elastic material such as natural rubber, synthetic rubber or a synthetic resin.
However, the electroconductive elastic member thus obtained has a relatively high electrical resistivity of 10.sup.15 to 10.sup.7 ohm/cm due to the employment of an ion or salt for electroconductivity, and the electrical resistivity thereof is quite unstable in long use.
To overcome these defects and to obtain the desired electrical resistivity, it is known to incorporate a fine electroconductive powder, such as of carbon black, aluminum, silver, nickel, copper or the like, into an elastic material in place of the aforesaid antistatic agent to obtain an electroconductive elastic member. However, incorporation of such fine electroconductive powder impairs the softness and pliability of the elastic member, rendering it hard and brittle and low in elasticity. The elastic member having such properties, when used in a form of roller or belt, possesses the disadvantages of high deficiencies in flexibility, elasticity, durability and transformability and thus fails to assure a uniform contact with relatively weak pressure against an opposing member with which it is urged into contact. Furthermore, when the electroconductive coating is formed over the surface of an elastic member, the coating is likely to separate from a base member thereby impairing the durability.
To assure a highly elastic, uniform contact pressure, an elastic sponge member having a low electric resistivity may be made by adding large amounts of a highly electroconductive material, such as metal powder and a foaming agent such as ammonium carbonate, diazoaminobenzene, hydrazide benzensulfonate or the like, to a synthetic rubber or like elastic material and foaming the resulting mixture, but the fine metal powder greatly reduces the elasticity of the resulting sponge product and its ability to effect intimate contact. Thus it is impossible to obtain a sponge-like elastic product having numerous uniform pores therein.
In the field of electrophotography, particularly in an electrophotographic copying apparatus of the electrostatic latent image transfer type, a metallic member or an electroconductive rubber member in the form of a roller has hitherto been used as an image transfer means. Specifically, these transfer means are either electrically grounded or applied with a bias potential and placed in contact with the surface of an image forming photosensitive plate with a copying paper inserted therebetween to transfer an electrostatic latent image to a copy paper.
However, with the use of a metallic roller as an image transfer means; (1) the phtosensitive plate bearing the latent image is subject to a heavy pressure of at least few kg/cm.sup.2 by the metallic roller for firm contact of the copy paper with the surface of the photosensitive plate resulting in damage to the plate. (2) The contacting area of the metallic roller with the surface of photosensitive plate is quite narrow and small due to the non-elastic nature of the metallic roller and this results in the uncertain migration of the electrostatic charges from the image bearing photosensitive member to the copy paper at the time of transfer. Accordingly, a non-uniform transfer is effected under this unstable and non-uniform transfer condition with a copy paper which is also subject to deformation caused by environment changes resulting from changes in temperature, humidity, etc. (3) A sudden and rapid injection of electrostatic charges to the copy paper is likely to occur at the initial stage of transfer and this will cause abnormal discharge phenomenon which results in the non-uniform transfer of the image.
To overcome these defects, the use of an electroconductive rubber roller has been proposed for this roller which is placed over a photosensitive plate with less force as compared with the metallic roller thereby overcoming the first defect mentioned above. Further, because of the elastic nature of this rubber roller, an electrostatic latent image is transferred under improved conditions. While this rubber roller is superior to the metallic roller in the above respects, it not only fails to overcome the third defect of the metallic roller mentioned above, but also has the following distinctive disadvantages in which; (1) a plasticizer added in the electroconductive rubber roller to bestow elasticity exudes at the surface of the roller consequent to environment changes and lapse of time, and tends to adhere on a copying paper or photosensitive plate to cause contamination thereon. (2) The elastic property of electroconductive rubber roller is poor in restorability subsequent to the release from compression contact with the opposing photosensitive plate, and the size of roller undergoes changes in long use which adversely affects the image transfer conditions. (3) The surface of electroconductive rubber roller is likely to be affected by ozone generated by the associated corona discharge device which ozone is transported thereto by the accompanying air flow caused by the rotation of a photosensitive plate and becomes tacky to adversely affect the copy paper and photosensitive plate.